Propped a bit differently, the .40 in your old Avistar will probably work. Almost any recent schnurle ported .40 would be a match for the best of the ubiquitous loop scavenged .40-.45's of the era when the Kaos was born.

If you have an LA .40 in the Avistar, you can get a cheap boost in power by fitting it with a Fox Stunt .35's muffler (the holes and exhaust port openings line up just fine and the Fox muffler is fairly cheap) and sawing off the back end of the Fox muffler so the opening is in the 1/2" diameter range. It'll be louder, but the power boost is something you'll feel visibly. OS's stock mufflers are fairly restrictive, but you can't just lop the rear end with a hacksaw like you can with Fox muffler due to that goofy through-bolt design of OS's.

Special bonus - the Fox muffler's don't seem to fall apart in flight with the frequency of that through-bolt design. Our field is littered with the back half of OS mufflers that fell apart in flight.